Improve exception message when retrieving String properties

If we have a property of type BINARIES and we try to read it, using the Oak API, as an array of STRING, then the exception message of the correctly raised IllegalStateException is:

java.lang.IllegalStateException: String is too long: 2325884045577463478" (where the number is basically random) 

In order to give a hint to the caller about what might go wrong, we should improve the exception message:

throw new IllegalStateException("String is too long: " + length + "; possibly trying to read a BLOB using getString; can not convert BLOB to String");